Generally, on the side of a DSLR lens, there is a switch that can be moved to either manual or auto. This determins the nature of the focus.
Generally, auto is sufficient, it is much faster, but not so precise. Below are situations when manual would be very useful.
When using autofocus, the camera selects which part to focus on, and with macro photography, if the camera selects the wrong part it could ruin the shot, so autofocus really helps. Use a tripod if possible, not using a tripod is likely to cause frustration, regardless of the mode.
Of course, there is nothing wrong with using autofocus for action, it is fast, and this is necessary for action, but for real precision, set up your tripod, position your camera and move your focus points.Keep your camera where the subject will enter the frame, and when it does… SNAP!
It is easy enought to take portraits where the whole face or body is in focus, but taking pictures of people at a more unusuaal angle, such as with only one eye, or only the nose in focus requires precision and care, and thats where manual focus comes in.
Glass– Taking pictures through it
When there are several subjects to focus on, especially when one is further afield makes things confusing for the camera. If it starts to flick focus quickly between both subjects, or have difficulty focusing on either one, it is time to call in Mr. Manual!
Glass– taking pictures of it
To snap this, choose a large aparture (low f stop) to ensure the depth of field is shallow (ensures the camera focuses on the glass) and position your AF points, then SNAP, SNAP, SNAP!
Many lenses find it hard to focus in low light, so manual helps alleviate the problem.